Humans make up a sliver of life on Earth, but our negative impact is massive

Of all the mammals on Earth, 60 percent are livestock and only 4 percent are wild animals.
(Photo: Smolina Marianna/Shutterstock)

When it comes to all the living things on our planet, humans make up a minuscule fraction. Although there are 7.6 billion people in the world, humans are a mere .01 percent of all organisms, according to a new study. We're well overshadowed by plants, bacteria and fungi.

Yet we've made a mighty impact. Since the start of humanity, people have caused the extinction of 83 percent of wild mammals and about half of all plants. Livestock kept by humans, however, continue to thrive. The authors estimate that of all the mammals on Earth, 60 percent are livestock.

"I was shocked to find there wasn’t already a comprehensive, holistic estimate of all the different components of biomass," lead author Ron Milo, at the Weizmann Institute of Science in Israel, told the Guardian. Milo said he now eats less meat due to the massive environmental impact of livestock on the planet.

"I would hope this gives people a perspective on the very dominant role that humanity now plays on Earth."

In the study, which was published in the Proceedings of the National Academy of Sciences, the researchers found that plants represent 82 percent of all organisms, followed by bacteria, which comprise about 13 percent. All other living things, including fish, animals, insects, fungi and viruses, make up only 5 percent of the world’s biomass.

The researchers calculated biomass (the total mass of all organisms) using information from hundreds of studies.

"There are two major takeaways from this paper," said Paul Falkowski, a biological oceanographer at Rutgers University who was not part of the research, told the Guardian. "First, humans are extremely efficient in exploiting natural resources. Humans have culled, and in some cases eradicated, wild mammals for food or pleasure in virtually all continents. Second, the biomass of terrestrial plants overwhelmingly dominates on a global scale — and most of that biomass is in the form of wood."

'We are changing the environment'

Light pollution takes a toll on all animals exposed to it. (Photo: EpicStockMedia/Shutterstock)

Wild species have been devastated by human practices such as hunting, overfishing, logging and land development, but the effect of our ever-closer presence on the animals around us may go deeper than we think.

Researchers at Arizona State University believe human activities may also be causing cancer in wild animals. They believe we could be oncogenic — a species that causes cancer in other species.

"We know that some viruses can cause cancer in humans by changing the environment that they live in — in their case, human cells — to make it more suitable for themselves," said study coauthor and postdoctoral researcher Tuul Sepp in a statement. "Basically, we are doing the same thing. We are changing the environment to be more suitable for ourselves, while these changes are having a negative impact on many species on many different levels, including the probability of developing cancer."

In a paper published in Nature Ecology & Evolution, the researchers say humans are changing the environment in a way that causes cancer in wild animals. Examples include pollution in oceans and waterways, radiation released form nuclear plants, exposure to pesticides on farmlands and artificial light pollution.

"In humans, it’s also known that light at night can cause hormonal changes and lead to cancer," Sepp says. "Wild animals living close to cities and roads face the same problem — there is no darkness anymore. For example, in birds, their hormones — the same that are linked to cancer in humans — are affected by light at night. So, the next step would be to study if it also affects their probability of developing tumors."

Now that the question has been raised, the researchers say the next step is to go into the field and measure the cancer rate in wild animal populations. If humans do indeed have a hand in wild animal cancers, then species may be more threatened than people think.

"To me, the saddest thing is that we already know what to do. We should not destroy the habitats of wild animals, pollute the environment, and feed wild animals human food," says Sepp. "The fact that everybody already knows what to do, but we are not doing it, makes it seem even more hopeless.

"But I see hope in education. Our kids are learning a lot more about conservation issues than our parents did. So, there is hope that the decision-makers of the future will be more mindful of the anthropogenic effects on the environment."